The invention relates to polypropylene films. In particular, the invention relates to polypropylene films that are clear and printable.
Polypropylene films are not printable. This limits their utilities in the areas where printable surfaces are necessary. Methods to improve printability of polypropylene are known, for example, flame treatment, plasma treatment, chemical treatment and corona discharge treatment. These treatments can soften or ionize the surface of polypropylene and make the surface temporarily printable.
U.S. Pat. No. 5,330,831 teaches how to modify a polypropylene substrate by corona discharge treatment and then cover the treated surface with a coating. The coating is a printable polymer and, therefore, the coated polypropylene is printable. Similarly, U.S. Pat. Nos. 5,789,123, 5,496,635, 5,496,636, 4,732,786, 5,827,627, and 5,789,123 also teach polypropylene films that are coated with printable coatings.
Another method to make polypropylene substrates printable is to make multilayer films. For example, U.S. Pat. No. 5,902,684 teaches a printable multilayer film that consists of a polypropylene layer, a primer coating layer, a layer of crosslinked polyvinyl alcohol, and a layer of a blend of polyvinyl alcohol and an ethylene-acrylic acid copolymer. A printable outer layer makes the multilayer film printable. Polypropylene films with multilayers or coated with printable coatings, although printable, are difficult and expensive to make. Moreover, these modifications often impair the clarity of polypropylene films.
Printable polypropylene films are needed. Particularly, printable polypropylene films having excellent clarity are needed. We have surprisingly found that a printable and clear polypropylene film can be produced from a blend of polypropylene and an ethylene-alkyl acrylate copolymer. More importantly, the films can be conveniently and inexpensively produced.
The invention is a clear and printable polypropylene film. The film is prepared from a blend comprising a polypropylene and an ethylene/alkyl acrylate copolymer. The ethylene/alkyl acrylate copolymer has a molar ratio of ethylene to alkyl acrylate from about 1/1 to about 20/1. The polypropylene contains up to about 20 wt % of recurring units of ethylene or a C4-C10 xcex1-olefin.
The invention also includes a clear and printable polypropylene film made from a blend comprising a polypropylene, an ethylene/alkyl acrylate copolymer and a third polymer. The third polymer is selected from polyester, acrylic resin, polyvinyl alcohol, polyvinyl chloride, polyvinyl acetate, polyvinyl ether, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, ethylene-acrylic acid copolymer, and mixtures thereof.
The invention includes a method to prepare a clear and printable polypropylene film. The method includes blending a polypropylene, an ethylene/alkyl acrylate and, optionally, a third polymer, and then converting the blend into a film.
The printable polypropylene film of the invention is made from a polymer blend. The polymer blend comprises from about 5 to about 55 wt % of an ethylene/alkyl acrylate copolymer. The ethylene/alkyl acrylate copolymer has a molar ratio of ethylene to alkyl acrylate within the range of about 1/1 to about 20/1. Preferably, the copolymer has a molar ratio of ethylene to alkyl acrylate within the range of about 5/1 to about 20/1. In general, the higher concentration of acrylate recurring units in the copolymer, the less copolymer is used in the blend.
Methods for preparing ethylene/alkyl acrylate copolymers are known. For example, U.S. Pat. No. 5,543,477, the teachings of which are herein incorporated by reference, teaches how to prepare ethylene/alkyl acrylate copolymer by free radical polymerization. Suitable ethylene/alkyl acrylates copolymers include copolymers of ethylene with a C1 to C20 alkyl acrylate or methacrylate. Examples are ethylene/methyl acrylate copolymers, ethylene/methyl methacrylate copolymers, ethylene/ethyl acrylate copolymers, ethylene/ethyl methacrylate copolymers, ethylene/butyl acrylate copolymers, ethylene/butyl methacrylate copolymers, and the like, and mixtures thereof. Ethylene/methyl acrylate copolymers and ethylene/butyl acrylate copolymers are preferred because of their low temperature flexibility. Ethylene/methyl acrylate copolymers are commercially available.
The polymer blend contains from about 45 to about 95 wt % of a polypropylene. Suitable polypropylene includes propylene homopolymers and random copolymers with ethylene or other xcex1-olefins. Propylene homopolymers can be isotactic, syndiotactic, or atactic, or mixtures theirof. Isotactic polypropylene is preferred because its crystalline nature gives the printable film better solvent and heat resistance.
Suitable random copolymers include propylene-ethylene copolymers and propylene-C4 to C10 xcex1-olefin copolymers. Examples of C4 to C10 xcex1-olefins are 1-butene, 1-hexene and 1-octene. Compared to propylene homopolymers, random copolymers feature increased clarity and improved impact resistance. Preferred random copolymers are propylene-ethylene copolymers. More preferably, the propylene-ethylene copolymers contain about 1 to about 20 wt % recurring units of ethylene. Most preferably, the propylene-ethylene copolymers contain about 1 to about 10 wt % recurring units of ethylene.
Polypropylene can be produced with Ziegler catalysts or single-site catalysts. Single-site catalysts can be divided into metallocene and non-metallocene. Metallocene single-site catalysts are transition metal compounds that contain cyclopentadienyl (Cp) or Cp derivative ligands (see U.S. Pat. No. 4,542,199). Non-metallocene single-site catalysts are referred to as those that contain ligands other than Cp but have the same catalytic characteristics as the metallocene. The non-metallocene single-site catalysts usually contain heteroatomic ligands, e.g., boraaryl (see U.S. Pat. No. 6,034,027), pyrrolyl (see U.S. Pat. No. 5,539,124), azaborolinyl (see U.S. Pat. No. 5,756,611) and quinolinyl (see U.S. Pat. No. 5,637,660). Single-site catalysts give polyolefins narrow molecular distributions.
Number average molecular weight of polypropylene is preferably within the range of 100,000 to 1,000,000, more preferably from 200,000 to 600,000. Molecular weight distribution (MWD) is the ratio of weight average molecular weight over number average molecular weight. Molecular weight and molecular weight distribution can be measured by GPC. Suitable polypropylene for use in the invention preferably has a MWD within the range of 1 to 30, more preferably from 1 to 10 as measured by GPC. Molecular weight can also be measured by melt flow rate (MFR). The lower the molecular weight of polypropylene, the higher the MFR. MFR can be measured by ASTM, e.g., ASTM D-1238-89. Suitable polypropylene preferably have a MFR within the range of 0.1 to 100, more preferably from 0.5 to 50 and most preferably from 1 to 10.
Optionally, the polymer blend contains a third polymer. Adding a third polymer into the blend can either enhance the performance of the product or reduce the cost. However, addition of a third polymer shall not significantly affect the printability or the clarity of the polypropylene film. Suitable third polymers include polyester, acrylic resin, polyvinyl alcohol, polyvinyl chloride, polyvinyl acetate, polyvinyl ether, ethylene-vinyl acetate copolymers (EVA), ethylene-vinyl alcohol copolymers (EVOH), ethylene-acrylic acid copolymers, and the like, and mixtures thereof. EVA and EVOH are preferred because they are easy to blend.
Optionally, the polymer blend also contains anti-oxidants, UV-absorbents, flow agents, or other additives. The additives are well known in the art. For example, U.S. Pat. Nos. 4,086,204, 4,331,586 and 4,812,500, the teachings of which are herein incorporated by reference, teach the UV stabilizers for polyolefins. Addition of the additives must not impair the printability or the clarity of the film of the invention.
Polypropylene films are not printable. The polypropylene films of the invention are rendered printable by uniformly blending a polypropylene with an ethylene/alkyl acrylate copolymer. Any suitable blending techniques can be used. The polymers can be blended in solution, in thermal processing such as extrusion, or in the polymerization reactor in-situ. By xe2x80x9cin-situ,xe2x80x9d we mean that adding a pre-prepared first polymer (e.g., an ethylene/alkyl acrylate copolymer) into the polymerization unit in which a second polymer (e.g., a propylene polymer) is made.
The polypropylene films of the invention can be prepared from the polymer blend by casting, press molding, blowing film extrusion, and many other suitable techniques. Blowing film extrusion is the preferred technique for a large commercial production. One advantage of the invention is that the film can be made in an existing instrument that is used to make ordinary polypropylene films.
The polypropylene film of the invention is clear and printable. By xe2x80x9cprintable,xe2x80x9d we mean that the film is printable without any surface treatment, although surface treatments, e.g., flame treatment, plasma treatment, chemical treatment and corona discharge treatment, may be employed to increase its performance.
The film has an excellent adhesion with commonly used printing inks. One advantage of the polypropylene film of the invention is that it is printable with both UV curable inks and waterborne inks. These ink systems have a minimum amount of VOC (volatile organic compound) solvents and therefore are environmentally safer.
The printed film has many applications. For example, it can be used for labels, documents, and advertisements. Because the film has an excellent clarity, it is particularly useful as labels for clear bottles, boxes, or other packages where goods inside the packages can be clearly seen without opening the packages.